Baccharis salicifolia, mule fat, is a widespread shrub to small tree occurring throughout California’s riparian habitats, and it is a critical nesting resource for willow flycatchers, an endangered species.  Under lower water availability or higher light regimes, mule fat individuals are shrubby with multiple stems per individual, but under higher water availability and lower light availability they produce fewer stems per individual, and their growth habit is a small tree.  Our study attempts to tease out the relative effects of water availability and light availability on this species.  We collected stems from a single population in the San Gabriel Mountains in Los Angeles County, and rooted them in a greenhouse.  Once the stems were rooted, we planted them in a mix of sand and soil, and placed the pots in large containers.  There were a total of four individuals in each of six treatments, flooded, intermediate water availability, and periodic water availability in a sun and shade.  Branch elongation and leaf production were measured monthly from June of 2008 through December of 2008.  Vulnerability curves for each individual were determined in February of 2009, and total leaf area was measured in October 2008 and February 2009. Specific Leaf Weight (SLW) was recorded in February 2009.

Results/Conclusions

Light availability had little influence on the total number of leaves produced, but did influence branching growth leading to fewer leaves per branch length on shaded individuals regardless of water availability.  Shaded individuals had significantly lower SLW, about 350.0 g/m², than individuals in the sun, 480.0 g/m², regardless of water availability.  Vulnerability to cavitation was influenced by light and to some extent water availability.  Shaded individuals in the flooded treatment had mean P₅₀ values of 2.75 MPa while sun individuals had mean P₅₀ values of 3.16 MPa.  All individuals in the periodic water availability in the sun died, while three out of four individuals in the shaded periodic water availability treatment lived.  Our results suggest that light availability is more important than water availability in determining the growth habit of this species.  These results also suggest that in the absence of disturbance, mule fat may drop out of the riparian woodland community as other species shade it.  Southern California’s streams and rivers are highly managed for flood control.  Our research suggests that this may complicate management of mule fat as a resource for nesting birds.